1. Field of the Invention
The field of the present invention is tandem type vacuum boosters of a type including a booster shell having a partition plate fixedly mounted therein for partitioning an interior of the booster shell into a front shell chamber and a rear shell chamber, a front booster piston provided to divide the front shell chamber into a fore-side front vacuum chamber and a back-side front working chamber, a front diaphragm superposed on a rear surface of the front booster piston, a rear booster piston provided to divide the rear shell chamber into a fore-side rear vacuum chamber and a back-side rear working chamber, a rear diaphragm superposed on a rear surface of the rear booster piston, the front booster piston and the front diaphragm being joined to the rear booster piston and the rear diaphragm through a piston boss which extends through the partition plate and leads to an output rod, a valve cylinder continuously formed with a rear end of the piston boss and slidably supported on a rear wall of the booster shell, an input rod for advancing and retreating movements, and a control valve for placing the two working chambers into alternate communication with the atmosphere and the two vacuum chambers in response to the advancing and retreating movements of the input rod, the input rod and the control valve being disposed within the valve cylinder.
2. Description of the Prior Art
Such a tandem type vacuum booster has already been proposed by the present assignee and is known, for example, from U.S. Pat. No. 4,905,573.
In the above tandem type vacuum booster, in order to connect the front and rear booster pistons and the front and rear diaphragms to the piston boss, three components: the piston boss, an end plate of a cylindrical connecting member connected to the front booster piston and superposed on a front end of the piston boss, and a retaining plate superposed on a rear end of the piston boss are coupled to one another by a plurality of through bolts, thereby fixing the front booster piston and the front diaphragm between the end plate and the front end of the piston boss and fixing the rear booster piston and the rear diaphragm between the retaining plate and the rear end of the piston boss.
In the above prior art tandem type vacuum booster, however, there is a problem that it is necessary to pass the plurality of through-bolts through the piston boss, the end plate and the retaining plate and tighten them. This results not only in an increased number of parts but also in necessitating much labor and time for assembling the parts.